Don't drop critical data messages

Assigning dataChannel sometimes happens after iceConnect.

Fixes symptom where Alice calls Bob.
Bob answers and sees the call UI as normal
but Alice sees call as continuing to ring

// FREEBIE

Signal/src/call/CallService.swift

@@ -1580,7 +1580,7 @@ protocol CallServiceObserver: class {
         self.peerConnectionClient?.setLocalVideoEnabled(enabled: shouldHaveLocalVideoTrack)
 
         let message = DataChannelMessage.forVideoStreamingStatus(callId: call.signalingId, enabled: shouldHaveLocalVideoTrack)
-        peerConnectionClient.sendDataChannelMessage(data: message.asData(), description: "videoStreamingStatus")
+        peerConnectionClient.sendDataChannelMessage(data: message.asData(), description: "videoStreamingStatus", isCritical: false)
     }
 
     // MARK: - Observers

Signal/src/call/PeerConnectionClient.swift

@@ -509,7 +509,15 @@ class PeerConnectionClient: NSObject, RTCPeerConnectionDelegate, RTCDataChannelD
 
     // MARK: - Data Channel
 
-    public func sendDataChannelMessage(data: Data, description: String, isCritical: Bool = false) {
+    // should only be accessed on PeerConnectionClient.signalingQueue
+    var pendingDataChannelMessages: [PendingDataChannelMessage] = []
+    struct PendingDataChannelMessage {
+        let data: Data
+        let description: String
+        let isCritical: Bool
+    }
+
+    public func sendDataChannelMessage(data: Data, description: String, isCritical: Bool) {
         AssertIsOnMainThread()
 
         PeerConnectionClient.signalingQueue.async {
@@ -519,9 +527,11 @@ class PeerConnectionClient: NSObject, RTCPeerConnectionDelegate, RTCDataChannelD
             }
 
             guard let dataChannel = self.dataChannel else {
-                Logger.error("\(self.TAG) in \(#function) ignoring sending \(data) for nil dataChannel: \(description)")
                 if isCritical {
-                    owsFail("\(self.TAG) in \(#function) ignoring sending \(data) for nil dataChannel: \(description)")
+                    Logger.info("\(self.TAG) in \(#function) enqueuing critical data channel message for after we have a dataChannel: \(description)")
+                    self.pendingDataChannelMessages.append(PendingDataChannelMessage(data: data, description: description, isCritical: isCritical))
+                } else {
+                    Logger.error("\(self.TAG) in \(#function) ignoring sending \(data) for nil dataChannel: \(description)")
                 }
                 return
             }
@@ -708,6 +718,14 @@ class PeerConnectionClient: NSObject, RTCPeerConnectionDelegate, RTCDataChannelD
             assert(self.dataChannel == nil)
             self.dataChannel = dataChannel
             dataChannel.delegate = self
+
+            let pendingMessages = self.pendingDataChannelMessages
+            self.pendingDataChannelMessages = []
+            DispatchQueue.main.async {
+                pendingMessages.forEach { message in
+                    self.sendDataChannelMessage(data: message.data, description: message.description, isCritical: message.isCritical)
+                }
+            }
         }
     }